Permanent magnet generators are typically characterized by the following advantages: simple construction, high efficiency, high power factor, and high power density. These types of generators are therefore well suited for direct-drive wind turbine applications.
As the output power of wind turbines has rapidly increased, the development of advanced generators has inherently involved the design and construction of large diameter and low speed machines. Over the past several years, high efficiency and lightweight generators using advanced magnetic materials have been developed.
There are several types of rotor topologies for permanent magnet (PM) synchronous generators. The configurations of the rotors can be divided into radial and transversal forms depending on the orientation of the magnetization direction of the permanent magnets (see for example FIGS. 1A and 1B of U.S. Pat. No. 6,879,075).
The transversal (or circumferential or tangential) configuration is more suitable for applications requiring high power density and performance. The transversal-oriented magnet configuration is characterized by a partially non-magnetic rotor core with alternating transversal-oriented permanent magnets and iron rotor pole pieces. The rotor pole pieces perform flux concentration, which improves the utilization of copper and iron of the stator. The stator may be the same as in a conventional radial oriented magnet generator design, but with reduced dimensions and weight, and lower losses.
One disadvantage of transversal-oriented magnet configurations as compared to radial-oriented magnet configurations is that the leakage flux may be comparatively larger. The leakage flux is the magnetic flux that does not cross the air gap, and that is lost in the rotor and therefore providing no useful magnetic field. The conventional transversal permanent magnet configuration thus leads to a poor utilization of magnets due to flux leakage through the rotor body.
A way to reduce these leakages is to use an intermediate ring made from non magnetic material, as described in document EP 2 028 744.
Another way to maximize the magnet utilization is to use trapezoidal magnets, as described in the aforementioned document U.S. Pat. No. 6,879,075 (see FIGS. 5 and 6).